Linear sensor arrays are used in printers to measure ink densities ejected by nozzles of a print head. The sensor array is mounted with reference to a light source and an image substrate, such as a print drum or belt. The light source includes a red light emitting diode (LED), a green LED, and a blue LED. The LEDs are operated independently so only one LED may be illuminated at a time. Alternatively, the sensor array, comprised of a set of red filtered photo sensors, a set of blue filtered photo sensors, and a set of green filtered photo sensors, may be illuminated with white light. The light from an illuminated LED is carried by a light pipe that has a length corresponding to the length of a sensor array, which operates as an uniformity sensor. The light is directed towards the image substrate, which typically is a rotating drum or belt. In these types of printers, an image is ejected onto the substrate and then transferred to a media sheet. The sensor array is positioned to receive the light reflected by the image substrate. When the substrate is bare, the light is reflected specularly and the response of the sensor array is large. When a test pattern is printed onto the image substrate and the light is directed towards this test pattern, the specular reflection is decreased, but the diffuse reflection increases. The differences in the specular reflectance measurements detected by the sensors in a sensor array for different test patterns are correlated to the densities of the ink on the substrate at the positions that reflected light into the sensors. These density measurements are used to adjust driving signals to the nozzles or to modify the input gray level spatially in an effort to present a printed pattern with uniform ink densities and correct registration.
The sensor array detecting the light reflected by the print drum includes a plurality of photosensitive devices that are typically arranged linearly. For example, a sensor array may be a plurality of charge coupled devices (CCDs) that are linearly aligned with 600 CCDs per linear inch. An 8.5 inch wide sensor array, consequently, has 5100 CCDs. Slight differences in the characteristics of each photosensitive device in an array cause the devices to respond differently to various amounts of illumination. That is, the signal generated by one photosensitive device in the array in response to a reflected light signal may differ from the signal generated by another photosensitive device in the array in response to the same amount of reflected light.
Another problem with the reflectance measurements obtained by sensor arrays is the structure of the image substrate. Many image substrates are rough and highly structured. Some sections of the surface reflect light more intensely into the light sensor, while other sections reflect light away from the sensor array and appear dimmer. In an effort to have the sensor array produce a more uniform response to reflected light, a two dimensional (2D) calibration process may be performed. The 2D calibration process begins with a capture of the sensor array response as the image substrate rotates underneath the sensor. This sensor array response is called a bare substrate response. The bare response is obtained by illuminating the substrate with the red light and obtaining the response with the light sensor, illuminating the substrate with the green light and obtaining the light sensor response, and illuminating the substrate with the blue light and obtaining the light sensor response. Alternatively, the bare response could be obtained by illuminating the image substrate with white light and then obtaining a response with the red-filtered sensors, the green-filtered sensors, and the blue-filtered sensors. After the bare response is measured, the 2D calibration process continues with a capture of the sensor array response without any illumination. This sensor array response is called the dark response. The final step of the 2D calibration process is to print a test pattern onto the image substrate and monitor the sensor array response as the image substrate rotates beneath the sensor array. This response is called the uncalibrated response. For each pixel in the test pattern, the difference between an uncalibrated pixel and the corresponding pixel in the dark response and the difference between a bare substrate pixel and the corresponding pixel in the dark response are computed. A calibrated image is then calculated as the ratio between the two differences at each pixel location.
In some systems, the linear array of photo sensors is not as wide as the image member width. In these situations, the sensor array must be moved to one or more different positions across the image substrate so a calibration may be performed at each sensor position. For example, an 8.5 inch sensor array may be used to capture an image of a twelve inch wide image substrate by mounting the sensor array on a carriage member so the sensor array is shifted 3.5 inches to capture the remaining portion of the print drum not captured in the first image. Of course, the first five inches of the second image overlaps with the last five inches of the first image. In this description, the first image is called a front print drum image and the second image is called a back print drum image.
In theory, the 2D calibration process should result in the front image of a drum being the same as the back image of the drum. In reality, this does not occur. As shown in FIG. 1, three reflectance measurements for front images of a drum onto which three test patterns have been printed are shown by curves 10, 12, and 14. Curve 10 is the reflectance measurement for the front image of the densest test pattern and curve 14 is the reflectance measurement for the front image of the least dense test pattern. The reflectance measurements for the three back images of the drum are shown by curves 16, 18, and 20. The overlap between the front images and the back images, which occur in the area denoted OA in FIG. 1, should be approximately the same because the same portion of the test pattern is being measured. Instead, the measurement curves are different and are evidence of an artifact in the measured responses caused by the sensor array. These response differences arise because the amount of diffusely reflected light collected across the sensor array varies. This collected light is caused by variations in the illumination system and focusing optics. The bare image substrate response collected during the 2D calibration process monitors only specularly reflected light so it cannot be used to calibrate these variations in diffusely reflected light. The higher the ink density in a test pattern, the more severe the sensor artifact becomes. The response difference is especially evident in the overlap area of the two measurements. Thus, the light sensor exhibits an artifact that affects the ink uniformity measurements for the printer. As shown by the curves in FIG. 1, the reflectance measurements for the higher density ink pattern portrayed in the curves 10, 16 reveal the sensor artifact more vividly than the reflectance measurements for the lower density ink pattern shown in the curves 14, 20 and 12, 18.